When I have imported or exported catalogs in LR, 99% of the time I do not want to pollute my catalog with whatever collections were in the other catalog. The only way to prevent this from happening is to open the new catalog, delete the collections, and then open the master catalog again. Likewise, I have never exported a catalog in which the collections were needed in the new catalog.

It would be a significant improvement in my workflow if there was an extra checkbox either on import or export or both to "include collections".